U.S. patent application number 11/650904 was filed with the patent office on 2007-07-12 for arsenic measurement using anodic stripping voltammetry.
This patent application is currently assigned to Rosemount Analytical Inc.. Invention is credited to Chang-Dong Feng, Joshua Xu.
Application Number | 20070158211 11/650904 |
Document ID | / |
Family ID | 38231705 |
Filed Date | 2007-07-12 |
United States Patent
Application |
20070158211 |
Kind Code |
A1 |
Feng; Chang-Dong ; et
al. |
July 12, 2007 |
Arsenic measurement using anodic stripping voltammetry
Abstract
Measurement of arsenic in an aqueous solution is provided. The
pH of the aqueous solution is adjusted to a pH of about 7.0 or
higher. The pH adjusted aqueous solution is then analyzed using
anodic stripping voltammetry to obtain an indication of a quantity
of arsenic in the solution. In one aspect, the pH is adjusted using
a phosphate buffer.
Inventors: |
Feng; Chang-Dong; (Long
Beach, CA) ; Xu; Joshua; (Irvine, CA) |
Correspondence
Address: |
WESTMAN CHAMPLIN & KELLY, P.A.
SUITE 1400, 900 SECOND AVENUE SOUTH
MINNEAPOLIS
MN
55402-3319
US
|
Assignee: |
Rosemount Analytical Inc.
Irvine
CA
|
Family ID: |
38231705 |
Appl. No.: |
11/650904 |
Filed: |
January 8, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60757458 |
Jan 9, 2006 |
|
|
|
Current U.S.
Class: |
205/775 ;
204/400 |
Current CPC
Class: |
G01N 27/42 20130101;
G01N 33/1813 20130101 |
Class at
Publication: |
205/775 ;
204/400 |
International
Class: |
G01N 27/26 20060101
G01N027/26 |
Claims
1. A system for quantifying the presence of arsenic in an aqueous
sample, the system comprising: a working electrode configured to
contact the sample; a second electrode configured to contact the
sample; a pH-adjusting material disposed within the aqueous sample,
the pH adjusting material configured to adjust the pH to at least
about 7.0; and an analyzer coupled to the working electrode and the
second electrode, the analyzer being configured to perform anodic
stripping voltammetry on the pH adjusted sample to determine an
amount of arsenic in the sample.
2. The system of claim 1, wherein the pH-adjusting material is a
buffer.
3. The system of claim 2, wherein the buffer is a phosphate
buffer.
4. The system of claim 1, and further comprising an additional
additive to the sample, the additive being configured to react with
at least one interfering metal.
5. The system of claim 4, wherein the additive is a chelating
ligand.
6. The system of claim 5, wherein the chelating ligand is
ethylenediaminetetraacetic acid.
7. The system of claim 1, wherein the working electrode is
constructed from a material selected from the group consisting of
gold, graphite, glassy carbon, and diamond thin film.
8. A method for measuring a quantity of arsenic in an aqueous
sample, the method comprising: adjusting the pH of the aqueous
sample to be at least 7.0; generating a negative potential on a
working electrode for a period of time; and reversing the potential
of the working electrode and measuring a current related to the
presence of arsenic in the sample.
9. The method of claim 8, wherein adjusting the pH of the aqueous
sample includes adding a buffer to the sample.
10. The method of claim 9, wherein the buffer is a phosphate
buffer.
11. The method of claim 8, and further comprising adding at least
one chelating ligand to the aqueous sample.
12. The method of claim 11, wherein the chelating ligand is
ethylenediaminetetraacetic acid.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is based on and claims the benefit
of U.S. provisional patent application Ser. No. 60/757,458, filed
Jan. 9, 2006, the content of which is hereby incorporated by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] Water is the most crucial element needed for human activity
on the planet, including agricultural, industrial and domestic use.
Unfortunately, water quality around the world is poor and getting
worse. While over 70% of the earth is covered in water, only about
0.01% is usable fresh water. And since water demand increases with
population, the re-use of water and proper treatment methods have
become a critical necessity.
[0003] Water is a universal solvent and comes in contact with a
vast array of harmful and/or undesirable substances. One of the
most dreaded contaminants in drinking water is arsenic. Arsenic
appears naturally in soil, water and bedrock. In pure form, arsenic
is a silver-gray or white brittle metal. Arsenic is virtually
tasteless and has no odor. Consuming less than a teaspoon full of
arsenic can generate severe headache, a tired feeling, confusion,
hallucinations, diarrhea, vomiting, digestive system bleeding,
seizures or a coma. Long term exposure to toxic forms of arsenic
are believed to cause various forms of cancer, such as skin, liver
or kidney cancer. Additionally, arsenic is known to be harmful to
the nervous system.
[0004] Accordingly, drinking water systems commonly measure arsenic
levels during processing. One promising technique for arsenic
measurement is believed to be the known analytical technique for
anodic stripping voltammetry (ASV).
[0005] Anodic stripping voltammetry is an electrolytic analytical
method in which a working electrode, in the case of mercury, is
held at a negative potential to reduce metal ions in solution and
form an amalgam with the electrode. The solution is stirred, or
otherwise agitated, to bring as much of the analyte(s) metal to the
working electrode as possible for concentration into the amalgam.
After the reduced analyte has accumulated for some selected period
of time, the potential on the working electrode is increased to
re-oxidize the analyte and generate a current signal. Other
electrode materials, such as graphite, glassy carbon, diamond thin
film, gold, et cetera can also be used as the working
electrode.
[0006] The increased potential can be in the form of a step
function, such as normal-pulse polarography (NPP) or
differential-pulse polarography (DPP). The concentration of the
analyte in the working electrode is generally a function of the
limiting current measured during the reduction of the metal; the
duration of the accumulation; the number of moles of electrons
transferred in the half reaction; the Faraday constant (96,487
coulombs/mole of e.sup.-) and the volume of the electrode. The
expression for the current produced by the anodic stripping depends
on the particular type of the working electrode, but is generally
directly proportional to the concentration of the analyte
concentrated into the electrode. One advantage of anodic stripping
voltammetry is the pre-concentration of the analyte into the
electrode, thereby allowing the method to achieve very, very low
detection limits.
[0007] In traditional anodic stripping voltammetry, the sample
solution is adjusted acidic by adding acid such as hydrochloric
acid (HCl), sulfuric acid (H.sub.2SO.sub.4), and/or nitric acid
(HNO.sub.3). In this acidic condition, all other metal ions in the
sample co-exist in cation form. However, some of the metal ions,
such as the copper ion, can interfere with the measurement of
arsenic in the anodic stripping voltammetry method. Such
interference can reduce the accuracy of such voltammetry
methods.
[0008] Providing analytical techniques for the accurate measurement
of arsenic in drinking water using anodic stripping voltammetry
would be beneficial to industries that provide, or otherwise
monitor, drinking water supplies for arsenic contamination.
SUMMARY OF THE INVENTION
[0009] Measurement of arsenic in an aqueous solution is provided.
The pH of the aqueous solution is adjusted to a pH of about 7.0 or
higher. The pH adjusted aqueous solution is then analyzed using
anodic stripping voltammetry to obtain an indication of a quantity
of arsenic in the solution. In one aspect, the pH is adjusted using
a phosphate buffer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a diagrammatic view of an arsenic measurement
system employing anodic stripping voltammetry in accordance with an
embodiment of the present invention.
[0011] FIG. 2 is a flow diagram of a method of measuring arsenic in
an aqueous sample in accordance with an embodiment of the present
invention.
[0012] FIG. 3 is a chart illustrating various examples of anodic
stripping voltammetry using square wave voltammetry of arsenic
(III) in a pH 7 phosphate buffer with a gold working electrode.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0013] FIG. 1 is a diagrammatic view of an arsenic measurement
system employing anodic stripping voltammetry in accordance with an
embodiment of the present invention. System 10 includes analyzer 12
coupled to a plurality of electrodes 14, 16 disposed within sample
solution 18. Electrode 14 is a working electrode and may be
comprised of mercury, or any other suitable metal, such as gold.
Sample specimen 18 contains a quantity of arsenic for which
quantification is desired. In accordance with embodiments of the
present invention, the pH of sample solution 18 is adjusted to
approximately 7.0, or higher. This is in distinct contrast to
anodic stripping voltammetry methods of the prior art that
generally acidify sample solutions by adding acids such as
hydrochloric acid, sulfuric acid, or nitric acid. Once the pH of
solution 18 has been suitably adjusted, electrode 14 is biased to a
negative potential by analyzer 12. The negative potential of
working electrode 14 causes the arsenic within solution 18 to
accumulate on electrode 14 by virtue of reduction. The working
electrode can be formed of any suitable material including, without
limitation, gold, graphite, glassy carbon, and diamond thin film.
In embodiments where electrode 14 is formed of gold, this
accumulation process forms reduced arsenic. Once suitable time has
passed, the potential between the plurality of electrodes 14 and 16
is reversed thereby causing oxidation of the arsenic accumulated
upon working electrode 14. The current observed during the
oxidation process provides information regarding the quantity of
arsenic present in solution 18.
[0014] Adjustment of the pH of sample solution 18 can be done in
any suitable manner. However, the pH is preferably adjusted by
utilizing a buffer solution. Preferably, the buffer is a phosphate
buffer, but other suitable buffers can be used in accordance with
embodiments of the present invention. In addition, other suitable
substances can be added to sample solution 18 to form complexes
with any interfering metals. For example, chelating ligands 10, can
be added to sample solution 18, such as ethylenediaminetetraacetic
acid. Adjusting the pH of sample solution 18 to be at or higher
than seven facilitates the formation of precipitates and complexes
with the interfering metals.
[0015] FIG. 2 is a flow diagram of a method of measuring arsenic in
an aqueous sample in accordance with an embodiment of the present
invention. Method 50 begins at block 52 where the aqueous
arsenic-containing sample is provided. Next, at block 54, the pH of
the sample is adjusted to equal, or exceed 7.0. Preferably, the pH
is adjusted using a buffer solution, such as a phosphate buffer.
Further, as indicated at phantom block 56, substances can be added
to the sample solution to form complexes with interfering metals.
For example, such substances can include chelating ligands, such as
ethylenediaminetetraacetic acid. Next, at block 58, anodic
stripping voltammetry is performed on the pH-adjusted aqueous
sample to determine and/or quantify the presence of arsenic in the
sample. At block 60, the measurement is provided.
[0016] FIG. 3 is a chart illustrating various examples of anodic
stripping voltammetry using square wave voltammetry of arsenic
(III) in a pH 7 phosphate buffer. The deposition time was 500
seconds. The plot is of the potential versus silver/silver
chloride, saturated potassium chloride.
[0017] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
* * * * *